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Stefan Arnórsson - One of the best experts on this subject based on the ideXlab platform.

  • geothermal systems in iceland structure and conceptual models i high temperature areas
    Geothermics, 1995
    Co-Authors: Stefan Arnórsson
    Abstract:

    Abstract There are 20 known high-temperature geothermal areas in Iceland and another eight potential areas. Surface manifestations are meagre in these eight areas and not conclusive, and no drilling has been carried out to prove or disprove the existence of high-temperature geothermal systems at depth. The high-temperature areas are located within the active volcanic belts or marginal to them. The heat source is considered to be magmatic, shallow level crustal magma chambers in the case of high-temperature systems associated with central volcanic complexes, but dyke swarms for the systems on the Reykjanes Peninsula where no central volcanoes have developed. Fossil high-temperature systems are abundant in Quaternary and Tertiary formations as witnessed by alteration of the basaltic eruptive rocks into lower-greenschist mineral assemblages. The fossil systems are typically associated with central volcanoes where intrusives account for 50% or more of the rock. The fossil systems are considered to have formed within the active volcanic belts but drifted out of them in conjunction with crustal accretion within these belts. In the process they may develop into low-temperature geothermal systems. Permeability is very variable within the drilled high-temperature areas, in the range 1–150 Millidarcies. The best permeability generally appears to be associated with sub-vertical fractures and faults. Permeability is poorest when the reservoir rock consists dominantly of intrusives, such as at Krafla, northeastern Iceland. It appears that intrusives are most abundant in reservoirs associated with central complexes that have developed a caldera. Temperatures follow the boiling point curve with depth, at least to the level of the deepest wells, in some areas, but in others they are lower. The highest recorded downhole temperature is >380°C. Hydrological considerations and permeability data favour that convection is density driven and that the source water is shallow groundwater in the vicinity of these systems. This groundwater is in most cases of meteoric origin. However, in three areas on the Reykjanes Peninsula it is largely or solely marine. The deuterium content of geothermal waters of meteoric origin is often lower than that of local precipitation. This has been taken to indicate that the source of supply is precipitation that has fallen on higher ground inland. This may indeed be the case, but flow from the source area is considered to be shallow. In some cases the low δD-values may stem from the presence of a component of an old water, which is isotopically lighter than today's precipitation at any particular site because the climate in Iceland was colder in the past. The geothermal seawater at Reykjanes and Svartsengi, southwestern Iceland, is considerably lower in deuterium than seawater. The cause of this is not known. However, reaction between seawater and basaltic rocks at very low temperatures may contribute, as well as rising of H 2 gas from deep levels and its reaction at shallower levels in the geothermal system to form water, but H 2 gas is much more depleted in deuterium than the associated water. Degassing of the magma heat source appears to add chemical constituents to the geothermal waters, such as boron, carbon and sulphur. Sometimes there may also be addition of Cl and H 2 O during events of recharge of new magma into the magma chambers in the roots of the geothermal system such as has been observed in the Krafla area. The high-temperature geothermal waters are close to chemical equilibrium with alteration minerals for all major components, except Cl and B. The alteration minerals typically display depth zoning because many of them are stable only over a limited temperature range. At temperatures above about 250°C the alteration mineral assemblage is that of the greenschist metamorphic facies. Precipitation of carbon as calcite and sulphur as sulphides, where boiling occurs in upflow zones of high-temperature geothermal systems, leads to strong enrichment of carbon and sulphur in the altered rock.

  • Geothermal systems in Iceland: Structure and conceptual models—I. High-temperature areas
    Geothermics, 1995
    Co-Authors: Stefan Arnórsson
    Abstract:

    Abstract There are 20 known high-temperature geothermal areas in Iceland and another eight potential areas. Surface manifestations are meagre in these eight areas and not conclusive, and no drilling has been carried out to prove or disprove the existence of high-temperature geothermal systems at depth. The high-temperature areas are located within the active volcanic belts or marginal to them. The heat source is considered to be magmatic, shallow level crustal magma chambers in the case of high-temperature systems associated with central volcanic complexes, but dyke swarms for the systems on the Reykjanes Peninsula where no central volcanoes have developed. Fossil high-temperature systems are abundant in Quaternary and Tertiary formations as witnessed by alteration of the basaltic eruptive rocks into lower-greenschist mineral assemblages. The fossil systems are typically associated with central volcanoes where intrusives account for 50% or more of the rock. The fossil systems are considered to have formed within the active volcanic belts but drifted out of them in conjunction with crustal accretion within these belts. In the process they may develop into low-temperature geothermal systems. Permeability is very variable within the drilled high-temperature areas, in the range 1–150 Millidarcies. The best permeability generally appears to be associated with sub-vertical fractures and faults. Permeability is poorest when the reservoir rock consists dominantly of intrusives, such as at Krafla, northeastern Iceland. It appears that intrusives are most abundant in reservoirs associated with central complexes that have developed a caldera. Temperatures follow the boiling point curve with depth, at least to the level of the deepest wells, in some areas, but in others they are lower. The highest recorded downhole temperature is >380°C. Hydrological considerations and permeability data favour that convection is density driven and that the source water is shallow groundwater in the vicinity of these systems. This groundwater is in most cases of meteoric origin. However, in three areas on the Reykjanes Peninsula it is largely or solely marine. The deuterium content of geothermal waters of meteoric origin is often lower than that of local precipitation. This has been taken to indicate that the source of supply is precipitation that has fallen on higher ground inland. This may indeed be the case, but flow from the source area is considered to be shallow. In some cases the low δD-values may stem from the presence of a component of an old water, which is isotopically lighter than today's precipitation at any particular site because the climate in Iceland was colder in the past. The geothermal seawater at Reykjanes and Svartsengi, southwestern Iceland, is considerably lower in deuterium than seawater. The cause of this is not known. However, reaction between seawater and basaltic rocks at very low temperatures may contribute, as well as rising of H 2 gas from deep levels and its reaction at shallower levels in the geothermal system to form water, but H 2 gas is much more depleted in deuterium than the associated water. Degassing of the magma heat source appears to add chemical constituents to the geothermal waters, such as boron, carbon and sulphur. Sometimes there may also be addition of Cl and H 2 O during events of recharge of new magma into the magma chambers in the roots of the geothermal system such as has been observed in the Krafla area. The high-temperature geothermal waters are close to chemical equilibrium with alteration minerals for all major components, except Cl and B. The alteration minerals typically display depth zoning because many of them are stable only over a limited temperature range. At temperatures above about 250°C the alteration mineral assemblage is that of the greenschist metamorphic facies. Precipitation of carbon as calcite and sulphur as sulphides, where boiling occurs in upflow zones of high-temperature geothermal systems, leads to strong enrichment of carbon and sulphur in the altered rock.

Stanley Ozimic - One of the best experts on this subject based on the ideXlab platform.

  • Petrological and petrophysical study of Permian arenites for potential subsurface storage of natural gas, Sydney basin, New South Wales, Australia
    The APPEA Journal, 2009
    Co-Authors: Stanley Ozimic
    Abstract:

    The expanding natural gas market in New South Wales during the next decades will require guaranteed back-up supplies in the event of insufficient deliverability through the Moomba-Sydney pipeline and/or cut-off of the pipeline. Subsurface storage of natural gas in natural reservoirs near the market area offers one solution for ensuring continuity of supply to this market. A petrological, wireline log, structural and reservoir-engineering study has been conducted of water-bearing arenites of the Permian Nowra Sandstone, Muree Sandstone and Snapper Point Formation in the Sydney Basin. This has resulted in the delineation of seven potential natural gas storage reservoirs near Sydney. The cap-rocks to the seven reservoirs are Permian impermeable arenites, siltstone, claystone and shale beds of the Berry Formation, Mulbring Siltstone, Wandrawandian Siltstone and Snapper Point Formation. Porosity in the Nowra and Muree Sandstone ranges from 5.5 to 12.2 percent and in the Snapper Point Formation from 5.4 to 6.8 per cent. Permeability is estimated to range from 0.47 to 5.00 Millidarcies. The structures of these potential reservoirs include both faulted and unfaulted, gently folded anticlines, and an irregular dome. Their areal extent and vertical closure range from 1 to 45 sq km and 20 to 225 respectively. Total potential storage capacity for the seven reservoirs is estimated to be 21.3 x 109m3 of natural gas, and their deliverability potential, based on permeabilities of 1.0 and 5.0 Millidarcies, ranges from 0.002 to 0.103 x 106m3 of natural gas per day per well.

C. H. Cheng - One of the best experts on this subject based on the ideXlab platform.

  • EFFECTS OF A BOREHOLE ENVIRONMENT AND RESIDUAL HYDROCARBON ON STONELEY WAVE AMPLITUDE AND REFLECTIVITY
    2016
    Co-Authors: Guo Tao, C. H. Cheng
    Abstract:

    In recent years, borehole Stoneley wave amplitude and reflectivity have been used for estimating formation permeability based on the strong correlation between Stoneley wave attenuation, reflectivity and formation fluid conductivity. There are other factors, however, that may cause substantial Stoneley attenuation and reflection in a borehole environment. To make better use of Stoneley measurements for formation permeability estimation, it is desirable to identify and quantify those causes of Stoneley attenuation and reflection that do not directly result from formation permeability. In this study, a simplified Biot-Rosenbaum model developed by Tang et at. (1991) is adopted to sys-tematically model Stoneley attenuation and reflection in various borehole environments and formation configurations. By changing pore fluid, formation porosity, lithology, bed boundaries and thickness in the modeling, the sensitivity of Stoneley wave propa-gation to these conditions are quantitatively assessed. It is found that the presence of a light hydrocarbon in the formation, especially a natural gas residual in the immediate vicinity of the borehole wall, even with only 5 % contained in pore fluid, may also cause substantial Stoneley attenuation and reflection. This phenomenon, on the other hand, can be used to evaluate a nonfractured, low permeability gas reservoir when combined with shear wave velocity data. For the full gas-saturated zone, Stoneley wave reflection may be observed even when the permeability is as low as a few Millidarcies. Com-pared to the effects of pore fluid, the effects due to lithology contrasts at the boundaries and the changes of nonfracture porosity are insignificant in the cases studied here. For a residual gas-bearing zone of moderate permeability, Stoneley wave attenuation and reflection may be observed if the zone is thicker than 0.5 meter

Cheng C. H. - One of the best experts on this subject based on the ideXlab platform.

  • Effects Of A Borehole Environment And Residual Hydrocarbon On Stoneley Wave Amplitude And Reflectivity
    Massachusetts Institute of Technology. Earth Resources Laboratory, 1996
    Co-Authors: Tao Guo, Cheng C. H.
    Abstract:

    In recent years, borehole Stoneley wave amplitude and reflectivity have been used for estimating formation permeability based on the strong correlation between Stoneley wave attenuation, reflectivity and formation fluid conductivity. There are other factors, however, that may cause substantial Stoneley attenuation and reflection in a borehole environment. To make better use of Stoneley measurements for formation permeability estimation, it is desirable to identify and quantify those causes of Stoneley attenuation and reflection that do not directly result from formation permeability. In this study, a simplified Biot-Rosenbaum model developed by Tang et at. (1991) is adopted to systematically model Stoneley attenuation and reflection in various borehole environments and formation configurations. By changing pore fluid, formation porosity, lithology, bed boundaries and thickness in the modeling, the sensitivity of Stoneley wave propagation to these conditions are quantitatively assessed. It is found that the presence of a light hydrocarbon in the formation, especially a natural gas residual in the immediate vicinity of the borehole wall, even with only 5% contained in pore fluid, may also cause substantial Stoneley attenuation and reflection. This phenomenon, on the other hand, can be used to evaluate a nonfractured, low permeability gas reservoir when combined with shear wave velocity data. For the full gas-saturated zone, Stoneley wave reflection may be observed even when the permeability is as low as a few Millidarcies. Compared to the effects of pore fluid, the effects due to lithology contrasts at the boundaries and the changes of nonfracture porosity are insignificant in the cases studied here. For a residual gas-bearing zone of moderate permeability, Stoneley wave attenuation and reflection may be observed if the zone is thicker than 0.5 meter.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumERL/nCUBE Geophysical Center for Parallel ProcessingUnited States. Dept. of Energy (Contract DE-FG02-86ER13636

L.w. Evans - One of the best experts on this subject based on the ideXlab platform.

  • Old wells, new concepts and a fractured reservoir: The Amoco, Champlin 242D No. 1, Echo Springs Area, Greater Green River Basin, Wyoming
    AAPG Bulletin, 1995
    Co-Authors: R.l. Billingsley, L.w. Evans
    Abstract:

    There is often much benefit to be gained in approaching older wells and productive areas from a different perspective. The Amoco Champlin 242D No. 1 was drilled in 1978 (sect. 11, T19N, R93W, Carbon Cty. WY.) and cored extensively in the Mesaverde Group for a petrophysically oriented formation evaluation. In addition, extensive bottom hole pressure build up (BHPBU) data was acquired to help design and assess completion practices. A lower Almond Fm. sand zone at 9656` MD was cored, perforated and tested extensively prior to and post hydraulic fracture treatment. Core analysis data indicates an unstressed, average air permeability of 0.75 Millidarcies for the matrix in zone. Attempts to {open_quotes}ballpark estimate{close_quotes} a stressed permeability give a figure of 0.2 Millidarcies, and comparison to limited relative permeability data from age equivalent strata supports a further reduction of 1 to 2 orders of magnitude. BHPBU permeabilities from the zone support an in situ effective permeability of 1-2 Millidarcies. A swarm of at least 3 natural fractures was observed in the cored interval that was tested. The inconsistency between the matrix and BHPBU data suggests the natural fractures may account for approximately 2 orders of magnitude difference between effective reservoir permeability and matrixmore » permeability alone. Studies of fracture direction, in situ stress and reservoir impact continue in this aging area of important production.« less